trinityamps.com

Hi

I have recently completed my first S111 build and wondered if someone could check my voltages as they seem a little lower than others I have seen.

I have used a Danbury DB1249 275 -190-0 160 mA transformer for this build which I bought from Ampmaker in the UK.

I opted for a solid state rectifier and used a 200 ohm sag resistor.

My voltages are :-

V1
1 135.1
3 0.84
6 129.5
8 0.88

V2
1 154.2
3 1.4
6 234.7
8 154.4

V3
1 192.2
2 46.1
3 65.4
6 182.5
7 45
8 65.3

V4
3 9.8
7 318.4
9 300.8

V5
3 9.9
7 318.9
9 300.6

240V AC B+ 403.5 with no tubes

The amp sounds good to me, but would there be any improvement in sound if the the voltages were closer to the standard values, and if so is there anyway to do this with the transformer that I am currently using.

Thanks

Jez

They do seem low. B+ is quite low at 318 VDC. 330 - 350 is closer.
B+ with no tubes should be well over 400.
Check that you are actually getting 190 X2 VAC at the rectifier with no tubes.

Specs at viewtopic.php?t=689

Sag resistor value ios good at 200 ohms but I would have thought with 190 - 0 - 190 on that tranny, the voltages would be higher. Mains is normal too.

I'm surprised actually but there's not much you can do to bring it up. One possibility is it's drawing more current than the PT can supply, but if it's 160 ma, should be fine there to.

Don't know.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Thanks Stephen for your prompt reply.

I will check the voltages again with another meter tonight just to verify that I am getting the correct readings, and will also check the current draw on the transformer.

Jez

Check the secondary VAC of the transformer.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I have checked the voltages with another meter and they seem to be about the same.

I disconnected the transformer secondary and got a reading of 285.8 V AC.

Jez

285 is close enough to 290, so the B+ should be higher (like 340 - 350) IMO IF the transformer can handle the total current.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I checked the B+ current with an ammeter and this was 92mA.

I then decided to take out the sag resistor and found that this brought the voltages much closer to the standard values.

The transformer is probably not quite up to the job.

Jez

Old post.. but just wanted to share,,
The Danbury transformer is a great cheap little one, but it is not rated at 160mA DC,,

That is 160mA AC which corresponds to 100mA DC at 100mA. Its home is the pp18 amp which is a 18 watt lite iib amp with no sag resistor,,
It has around 345V in the PP18 (depending on your wall outlet) with a 150r bias resistor and 1.5K dropping resistor and a shared 1K screen resistor for the two EL84 (which is like having 2K resistors on each EL84 tube),, This is because the screen voltage is above the 300V rating and Barry at Ampmaker.com wants your tubes to live a good life..

The PP18 draws around 85-90mA at idle and maxes around 105mA on peaks..

With a EZ81 rectifier you want something a little stronger like the classictone 18 watt marshall PT. That one is rated at 290VDC/120mA. It is a lot stronger and has more 6.3V filament current also..

My experience with these modern small transformers is that excessive filament draw will heavily affect the high voltage capacity.. They might say that the 6.3V line is rated at 3A but load it with 2A and it will run much cooler..

I have a small Laney cub 12r with 3 12ax7 and two jj EL84 tubes and the power transformer ran excessively hot after one hour at idle,, And that was a grid bias amp with 22mA/300V (55% of idle dissipation)..
I unplugged the first one preamp tube and the plate voltage increase a little,, took out another 12ax7 and the power transformer ran much cooler,, that was with a 1.95A filament draw instead of 2.55A with all tubes in...

The same transfomer comes in ampmakers double six amp.. (1 12ax7 + 2 6V6GTA) tubes in parellel class A with 560r resistors on the power tubes with ~ 350V on the 6v6 plates..

He substituted the 6v6 with 6l6 tubes and lowered the bias resistors to 270r pr 6l6 tubes for 19-20 watts of total cathode current on each tube.. The power transformer gets a little hot and the amp produces around 13.5W class A output..
Not bad for such a low rated power transformer. Heater draw is (0,9+0,9+0,3= 2.2A)